Mind, Body, World- Foundations of Cognitive Science, 2013a

old clocks and gas meters” (Grey Walter, 1963, p. 244). By 1951, these two had been
replaced by six improved machines (Holland, 2003a), two of which are currently
displayed in museums.
The robots came to be called Tortoises because of their appearance: they seemed
to be toy tractors surrounded by a tortoise-like shell. Grey Walter viewed them as
an artificial life form that he classified as Machina speculatrix. Machina speculatrix
was a reaction against the internal variability in Ashby’s Homeostat. The goal of
Grey Walter’s robotics research was to explore the degree to which one could produce
complex behaviour from such very simple devices (Boden, 2006). When Grey Walter
modelled behaviour he “was determined to wield Occam’s razor. That is, he aimed
to posit as simple a mechanism as possible to explain apparently complex behaviour.
And simple, here, meant simple” (Boden, 2006, p. 224). Grey Walter restricted
a Tortoise’s internal components to “two functional elements: two miniature radio
tubes, two sense organs, one for light and the other for touch, and two effectors or
motors, one for crawling and the other for steering” (Grey Walter, 1950b, p. 43).
The interesting behaviour of the Tortoises was a product of simple reflexes that
used detected light (via a light sensor mounted on the robot’s steering column) and
obstacles (via movement of the robot’s shell) to control the actions of the robot’s two
motors. Light controlled motor activity as follows. In dim light, the Tortoise’s drive
motor would move the robot forward, while the steering motor slowly turned the
front wheel. Thus in dim light the Tortoise “explored.” In moderate light, the drive
motor continued to run, but the steering motor stopped. Thus in moderate light
the Tortoise “approached.” In bright light, the drive motor continued to run, but the
steering motor ran at twice the normal speed, causing marked oscillatory movements.
Thus in bright light the Tortoise “avoided.”
The motors were affected by the shell’s sense of touch as follows. When the
Tortoise’s shell was moved by an obstacle, an oscillating signal was generated that
first caused the robot to drive fast while slowly turning, and then to drive slowly
while quickly turning. The alternation of these behaviours permitted the Tortoise to
escape from obstacles. Interestingly, when movement of the Tortoise shell triggered
such behaviour, signals from the photoelectric cell were rendered inoperative for a
few moments. Thus Grey Walter employed a simple version of what later would be
known as Brooks’ (1999) subsumption architecture: a higher layer of touch processing
could inhibit a lower layer of light processing.
In accordance with forward engineering, after Grey Walter constructed his
robots, he observed their behaviour by recording the paths that they took in a number
of simple environments. He preserved a visual record of their movement by using
time-lapse photography; because of lights mounted on the robots, their paths were
literally traced on each photograph (Holland, 2003b). Like the paths on the beach
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